Copper (Cu) has emerged as an alternative interconnect metal to conventional aluminum in integrated circuitry (IC) device fabrication due to its low resistivity and good electromigration properties. Several deposition techniques, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), and electrochemical plating have been used for copper “damacene” interconnect fabrication. Both PVD and plating methods suffer from difficulty in filling copper in narrow and deeply trenched structures and such methods form voids that lead to defect formation. CVD techniques, which offer good conformal coverage, are used to provide copper seed layers in order to enhance nucleation of copper growth and suppress void formation for sub-0.18 micron scale IC device fabrication.
In prior art CVD methods, copper precursors of the type: (hfac)CuL, where hfac represents hexafluoroacetylacetonate and L represents a neutral ligand, have been typically used. Copper precursors of the type of (hfac)CuL undergo “disproportionation reaction” at temperatures below 200° C. to provide copper thin films, as shown in the following equation:2(Hfac)Cu(+1)L→Cu(+0)+(Hfac)2Cu(+2)+2LAlternatively, (Hfac)2Cu is used as a copper precursor in metal-organic chemical vapor deposition (MOCVD) for copper films. However, MOCVD using (Hfac)2Cu precursor requires relatively high deposition temperatures (such as above 400° C.) and hydrogen (H2) gas as a reducing agent.
Another problem of prior art techniques for CVD of copper films using hfac-based copper precursors is the incorporation of trace amount of fluorine atoms at the interface between the substrate and copper layer, which degrades adhesion of the copper layer to the substrate. This causes problems when the deposited copper films undergo a chemical mechanical polishing (CMP) step to fabricate the copper interconnect structure in the back-end-of-line IC processing. Accordingly, further development of deposition techniques and processes of copper films is needed.